The cenozoic history of volcanism and hydrothermal alteration in the central Andean flat-slab region: New Ar-40-Ar-39 constraints from the El Indio-Pascua Au (-Ag, Cu) belt, 29 degrees 20 '-30 degrees 30 ' S

Ninety-three new Ar-40-Ar-39 laser step-heating plateau dates for igneous r ocks and alteration minerals from the El Indio-Pascua Au-Ag belt permit sig nificant refinement of the Tertiary volcanic stratigraphy and the definitio n of a succession of alteration events in this major mineralized district. Eight distinct Tertiary stratigraphic units are now recognized (two newly d efined in this research): (1) the 30-36 Mia Bocatoma Unit, comprising diori tic and granodioritic shallow-level intrusions; (2) the voluminous 23-26 Ma Tilito Formation, consisting predominantly of dacitic tuffs; (3) the 17.5- 21 Ma Escabroso Group made up of extensive successions of andesitic flows a nd coeval hypabyssal intrusions; (4) the 14-17 Ma, andesitic, Cerro de las Tortolas Formation and its intrusive lithodeme, the Infiernillo Unit; (5) t he dacitic, 11.0-12.7 Ma, Vacas Heladas Formation; (6) the rhyodacitic 7.5- 8 Ma Pascua Formation, defined in, this study; (7) the 5.5-6.2 Ma rhyolitic Vallecito Formation; and (8) the recently defined rhyolitic 2 Ma Cerro de Vidrio Formation. Magmatic activity decreased markedly following the erupti on of the Cerro de las Tortolas Formation. Hydrothermal activity occurred at least from the late Eocene to the Late Mi ocene, but economic Au-Ag-Cu mineralization was confined to the 6-9.5 Ma in terval, the only observed contemporaneous igneous unit being the restricted Pascua Formation. Epithermal Au-A-Cu deposits and major prospects emplaced in this period include, from north to south, Pascua-Lama, Veladero, Sancar ron, Rio del Medio, El Indio, Tambo, and Vacas Heladas. The widespread, alb eit barren. alteration associated with the Bocatoma, Escabroso, Infiernillo , and Vacas Heladas magmatism indicates that the availability of hydrotherm al fluid was not the controlling factor for ore formation, emphasising inst ead the role of the metal content of the magmas associated with epithermal mineralization, and/or the requirement for favorable physiographic conditio ns at the site of ore deposition.